The invention relates to a method of compressing data obtained from ultrasonic measurements made by an ultrasonic sonic pipe inspection probe which detects corrosion or other abnormalities in a pipe wall. The data are derived from ultrasonic propagation time measurements which are obtained by ultrasonic transducers combined in a module. The module or modules are disposed on predetermined circumferential areas of an ultrasonic pipe inspection probe with which they slide along the inner surface of a pipe to be checked. The ultrasonic transducers emit an ultrasonic pulse with a repetition frequency adjusted to the probe speed and receive the echo returned from the inner and outer surfaces of the pipe. By determining the travel time difference and taking into consideration the sound speed, the remaining thickness of the pipe can be determined over its whole surface. Because of the signal/statics ratio preferably the first received echoes are utilized.
The data derived from the measurements are recorded in a data storage during the travel of the ultrasonic inspection probe. However, the amount of data is very large. Without data compression the limits of the data storage are rapidly reached which substantially limits the travel distance of the ultrasonic inspection probe.
Normally, however, it can be assumed that the walls of a pipeline are essentially sound, that is, in good shape, over long distances. In those areas of course not all the measurements need to be stored but the tolerable measurement values can be replaced by a representative value. Upon retrieval of the data from the data storage after completion of the travel through the pipeline, the condition of the pipeline should be represented without any gaps.
DE 3638936 discloses a method wherein the data are compressed in such a way that measurement values deviating from a previous measurement value within a predetermined range (tolerance band) are only counted and recorded and stored by numbers. However, with the predetermined range this method does not or does not sufficiently reduce recording requirements when the wall thickness changes or the sensor mounting structure is worn, since, in those cases, the measurement values are outside the predetermined range. Furthermore, with this method the predetermined range may fall into a measurement value range which is important or corrosion determination whereby the quality of the quantitative corrosion determination is reduced.
Known methods of data compression are described and explained in DATACOM/91, pages 88 ff. The most suitable method is, or example, the Huffmann-coding (see page 94, columns 2 and 3 and page 96, columns 1 and 2). However, these methods do not take into consideration the physical laws of ultrasonics. Consequently no great savings are achieved therewith.
Ultrasonic data are recorded, for example, in a 12-bit data format. The first stage of the compression includes a data reduction. From the 12-bit data format the information is selected which is needed for later corrosion determination. For this a 7-bit data format is sufficient which is a subquantity of the 12-bit data format. This data format covers a defined value range with a predetermined resolution.
Each set of data comprising 64 advancement and wall thickness values entered into a compression computer is taken as a data set and compressed. The compression is based on the assumption that pipelines are free of corrosion over long distances and measurement values taken do not distinguish or are within a narrow tolerance band. This tolerance band is generated by arranging a window of predetermined width around a previously determined reference value. With the tolerance band the roughness of the wall and irregularities in the manufacture of the pipe are suppressed. By means of this tolerance and compression window the data are recorded with two different resolution ranges: a fine resolution for the corrosion areas and a rough resolution in the areas of normal wall thickness or nominal advancement.
The principle of the data compression method resides in the representation of multiple consecutive data, which are within the tolerance band, by means of a multiplier. The 7-bit data representation permits assignment of a special meaning to the eighth bit of a data word. The eighth bit is utilized as a prefix bit in order to be able to change the interpretation of the data word. The first seven bits may then contain a multiplier rather than a measurement value whereby it contains the number of measurement values which are within the predetermined tolerance range which are formed around the preassigned reference value. Maximally this may be 128 such values which corresponds to a binary representation of 7 bits. If a measurement value is outside the window, its value is fully recorded. The prefix bit is then not utilized. The multiplier count starts at 80H wherein 80H corresponds to a multiplier of 1. In the most advantageous case, when all measurement values are within the tolerance bands consequently only one byte with the value of 255 (=FFH) is recorded. It is the most disadvantageous case when all measurement values are outside the tolerance bands or one or more values outside the tolerance band are always followed by a value within the tolerance band. In both these cases, 128 bytes have to be recorded.
It is the object of the present invention to increase the data compression in the data storage of an ultrasonic pipe inspection probe so that it can pass through long stretches of pipelines without limitations and efficiently while the rough environmental conditions are particularly taken into consideration for data storage.